how can i focus textbox in xaml with keybinding?

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WPF

how can i set focus on a textbox when F3 key pressed

in other words,do some thing like bellow :

private void Window_PreviewKeyDown(object sender, KeyEventArgs )
{
       switch (e.Key)
       {
           case Key.F3:
               myTextBox1.Focus();
               break;
           case Key.F4:
               myTextBox2.Focus();
               break;
           default:
               break;
      }
}

note:I want to do it in xaml.